Multilayered ceramic structures may be used to form electronic devices such as ceramic capacitors, multilayered ceramic integrated circuits (MCIC), multichip modules, integrated circuit packaging, high temperature sensors (such as exhaust gas sensors), fuel cells, and fuel cell reformer systems. Multilayered structures also find application in transmit/receive modules in phased array radars. These devices may be used as substrates to support and interconnect electronic components mounted thereon, and, to this end, may include open cavities on one or more surfaces for receiving the electronic components.
Such multilayered structures are often made by laminating together layers or sheets of unfired ceramic tape, known in the art as “green-tape,” and then firing the laminated layers to form a finished structure. Green tape is commercially available, for example, from the DuPont Company under the product designation #951AT. The tape contains a material formulation which can be a mixture of glass and ceramic fillers which sinter at about 850° C., and exhibits thermal expansion similar to alumina. Low-temperature processing permits the use of air-fired resistors and precious metal thick film conductors such as gold, silver, or their alloys.
In electronic applications, one or more of the green-tape layers may include metallized portions to provide conduction pathways for electrical current in the finished multilayered structure. The green-tape layers may also have portions punched out to define vias, channels, or cavities. A method of forming cavities in a multilayer LTCC substrate is disclosed, for example, in U.S. Pat. No. 5,855,803, entitled “Template Type Cavity-Formation Device for Low Temperature Cofired Ceramic (LTCC) Sheets” which patent is hereby incorporated by reference.
LTCC structures may be formed with open cavities on one or more faces in which integrated circuit devices or other circuit elements can be mounted. Laminating layers of green tape to form such LTCC structures requires that a constant pressure be applied to the stacked sheets of green tape. This is a relatively straightforward process when the top and bottom surfaces of the tape stack are planar. When the outer surfaces include cavities for mounting electronic components, however, it becomes difficult to apply an even pressure to both the outer surface and the floors of the cavities.
One method for applying an even pressure during a lamination process includes the use of silicone molds having a pattern of projections complimentary to a pattern of cavities on the stack of green tape layers so that the projections extend into the cavities and apply an even pressure to the floors of the cavities. Another method, disclosed in U.S. Pat. No. 5,683,535, entitled “Method and Apparatus of Producing Cavities in LTCC Substrates” and incorporated herein by reference, uses a flexible rubber sheet that conforms to the shape of the cavities when pressure is applied.
Such prior art methods work reasonably well for structure having cavities in only one surface. In these conventional methods, the structures are placed against a flat, substantially rigid, back plate while pressure is applied to the surface of the structure having cavities. However, when forming an LTCC structure having cavities in two opposing sides using known methods, a flat back plate cannot be used because this method tends to deform the cavity floors. Sometimes the floors are bowed, sometimes punctured, and in most cases a finished structure is produced that has a floor too distorted to be useful. Such floors may have a flatness variation greater than 2 mils per inch, for example; a flatness variation of less than or equal to about 1 mil per inch is required for many applications. It is therefore desirable to provide a method of increasing the flatness of portions of LTCC structures. It is also desirable to provide a method of forming LTCC structures having at least one cavity in each of first and second opposing faces that produces cavity floors with a flatness variation less than about 1 mil per inch.